Polybutylene Naphthalate (PBN) is a polyester of the naphthalate polyester family. It has excellent wear resistance, sliding characteristics, hydrolysis resistance, and chemical resistance. Compared to its analogous polyester, it has superior gas barrier properties, excellent chemical and hydrolysis resistance, excellent sliding performance, high deflection temperature under load and faster crystallization making it superior over other polyesters such as polybutylene terphthalate, polyethylene naphthalate, polyethylene terphthalate, and Polyethylene terephthalate glycol-modified (PET-G).
PBN polyester has been used in a variety of applications because of its better wear properties than Polyphenylene Sulfide (PPS) and polyacetal in addition to excellent chemical resistance, superior barrier properties, and superior sliding characteristics.
When meshing of gears made from the same material is undesirable, PBN is used and its current commercial application include components in gear-change levers. Due to its superior gas-barrier properties. It is utilized as backsheet material for solar battery chargers where a service life of less than 10 years would be anticipated and the charger would typically be used indoors. Backsheets are normally comprised of multilayer structures. PBN can further be used to make fuel tanks, hose and fuel transporters.
The applications of PBN in packaging are very limited so far because of its fast crystallization characteristic that results in opaque container, but at the same time its superior gas barrier properties makes it one of the most preferable polyester for containers, hoses, fuel storage and transporter.
Amorphous polymer can be crystallized by thermal crystallization while cooling from the melt and heating solidified polymer above ambient temperature, and mechanical stretching (Strain induced crystallization). Crystallization affects optical, mechanical, thermal and chemical properties of polymers depending upon the size of crystals. The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10% and 80%, thus crystallized polymers are often called “semicrystalline” materials.
The properties of semicrystalline polyesters are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains.
It is therefore desirable to achieve the proper crystallization of polybutylene naphthalate polyester to improve their optical, barrier, mechanical, and thermal properties. Semicrystalline polymers usually appear opaque because of light scattering on the numerous boundaries between the crystalline and amorphous regions. The rate of crystallization is estimated by the half time crystallization method. The half time crystallization (T1/2) is the half time required to achieve the crystallization of melt polymer while stretching or heating. The more value of the T1/2 means slow rate of crystallization and vice-versa. T1/2 for polyethylene terephthalate (PET) is 18 seconds, for polyethylene naphthalate (PEN) is 115 seconds, for PBT is 3 seconds and for PBN is 1 second. Thus, due to fastest rate of crystallization, polybutylene naphthalate (PBN) is always found opaque in appearance which makes it undesirable for use in manufacturing transparent containers.
It is desirable to design and produce a modified polybutylene naphthalate polyester by controlling its crystal size and rate of crystallization, as the controlled crystallization of polybutylene naphthalate further enhances its transparency.
There has been a disclosure of the process of suppressing the crystallization rate of polybutylene naphthalate. Reference is made to U.S. Pat. No. 6,451,966 wherein a process to suppress the rate of crystallization of polyesters such as PET, PBT, PEN, PBN in Solid State Polymerization (SSP) is disclosed. In this process, the incorporation of low concentration of isophthalaic acid and ethylene glycol has been disclosed to suppress the rate of crystallization during SSP process so as to achieve better SSP rate. The patent ‘966’ does not disclose any increase in crystallinity and transparency of polybutylene naphthalate and limits its effect to transparency of product made of polybutylene naphthalate polyester. It does not provide solution of removing haziness in container made of polybutylene naphthalate copolymer.
Reference may be made to U.S. Pat. No. 5,612,423. which suggest a process to prepare crystallizable copolymers such as PET, PBN, with high molecular weights and high melting points. Here, the Solid State Polymerization (SSP) is continued until the Intrinsic Viscosity (I.V.) of the polymer reaches any desired level. Thus, the claimed process controls the rate of crystallization on the basis of I.V. characteristics. Further, it does not disclose the use of isophthalaic acid, monoethylene glycol or diethylene glycol or cyclohexanedimethanol (CHDM) as comonomer. The said process, however, does not teach about the transparency and improved barrier properties of the final product. In said patent, there is no reference of transparency and barrier properties of polybutylene naphthalate while making container.
However, there are not many reports which specifically described a modified polybutylene naphthalate. Hence, there is an urgent need to produce modified polybutylene naphthalate polyester with enhanced transparency and improved properties. The present invention overcomes the problem of loss of transparency in polybutylene naphthalate due to its fast crystallization of polybutylene naphthalate by achieving the proper crystallization of polybutylene naphthalate.
The crystallization rate and growth of crystal size is controlled by slightly retarding the rate of crystallization. The slight retardation in the rate of crystallization promotes the requisite growth of nucleation thereby limiting the size of the crystallites and ensures transparency along with increase crystallinity making them usable in packaging application for transparent containers in both monolayer as well as multilayer containers.